The present invention relates generally to automatic eye tracking wherein the tracking precision is enhanced based on an optical-transfer-function modifying mask, which enables the eye tracker to work within a relatively large range of distances. More particularly the invention relates to a system according to the preamble of claim 1 and a method according to claim 14. The invention also relates to a computer program according to claim 23 and a computer readable medium according to claim 24.
The concept of eye tracking is well known in the art, and a number of different techniques have been developed for accomplishing automatic eye and gaze tracking. In the area of remote, non-obtrusive eye tracking, the most commonly used designs are based on pupil center corneal reflection (PCCR) methods. The basic idea behind this approach is to use at least one light source, and by means of a camera, capture a series of images of the eye. In each image the light source's reflection, the glint, in the cornea and the pupil are identified. A vector defined from the glint to the center of the pupil is then used to estimate the eye's gaze direction. Furthermore, within the PCCR-eye-tracking field there exist two main strategies to identify the pupil in the above-mentioned images. The light source may be positioned as close as possible to the camera's optical axis. This results in that a part of the eye's retina illuminated by the light source reflects light back into the camera, and hence the pupil appears bright in the registered images. Tracking solutions based on this strategy are therefore categorized as bright-pupil (BP) PCCR. Alternatively, the light source can be positioned at a distance from the camera's optical axis. As a result, essentially no light from the light source will be reflected via the retina into the camera, and the pupil appears dark in the registered images. Tracking solutions based on this strategy are therefore categorized as dark-pupil (DP) PCCR.
Whether BP- or DP-PCCR is preferable depends on i.a. the ambient light conditions, the subject's age and gender because these factors influence the pupil area. Moreover, the BP response is highly influenced by the ethnicity of the person whose eyes are being tracked. For instance, it has been found that Hispanics generally have a very strong BP response and Caucasians have a somewhat weaker BP response, however still fair enough. Nevertheless, Asians in many cases have an in adequate BP response. Hence, in order to ensure a reliable eye tracking, a combination of BP- and DP-PCCR tracking is often desirable.
The published International Patent Application WO 2004/045399 describes a system wherein the eyes' positions and gaze directions are detected and tracked. The system includes a camera and a number of light sources, which are distributed around a display, e.g. a computer screen. By sequentially illuminating a subject viewing the display with light from different light sources it is possible to alternatively detect the eyes' position and the gaze direction. However, in order to perform this evaluation, the camera must generate data of high image quality. This, in turn, requires high-class optics, a high-performance image sensor and/or well-controlled light conditions. It is also very important that the subject's eyes remain in focus during the tracking procedure. To this aim, the camera must either be equipped with an auto-focus arrangement, or operate with an optical system that has a comparatively small numerical aperture (i.e. a high F-number) to accomplish a sufficiently large depth of field. The former alternative renders the camera complex, heavy and expensive, while the latter alternative further increases the performance requirements on the image sensor, which is a parameter that also translates into cost.
The U.S. Pat. No. 5,748,371 discloses a system for increasing the depth of field and decreasing the wavelength sensitivity and the misfocus-producing aberrations of the lens of an incoherent optical system. This technique is referred to as wavefront coding. Here, a special purpose optical mask is included in the incoherent optical system. The mask alters the optical transfer function, such that it remains essentially constant within some range from the in-focus position of the unaltered system. Signal processing of a resulting intermediate image undoes the optical transfer modifying effects of the mask, which provides an in-focus image over an increased depth of field. Although this system is efficient in terms of enabling a long focus range based on relatively simple and low-cost optics and sensors, the design is not well suited for direct implementation in an automatic eye tracking system. Namely, here, eye-tracking specific image parameters, such as eye positions and gaze directions, must be derivable with very high accuracy, whereas essentially all other image data may be discarded. For example, when a rough estimation of the eyes' position has been made the eye-tracking camera normally zooms in (optically, or digitally) towards this position, and/or selects a so-called region of interest (ROI) on the image sensor around this position, to improve the gaze tracking precision and/or reduce the data rate to the image processor. Nevertheless today, there is no wavefront-coding based design, which is adapted to enable any operations of this kind.